We can make an equation to figure out the answer. This is a quite common equation with a flat fee and a rate above that. The equation will be the same as the line slope formula: y = mx + b.

In this case, we can say the equation is

Total = (price per gallon)(# of gallons) + flat fee

so

40.50 = .0075g + 13.50 (where g is # of gallons)

then

27 = .0075g

then

27/.0075 = g

cancel 3 from top and bottom:

9/.0025 = g

Multiply top and bottom by 4 to make the decimal easy:

36/.01

multiply top and bottom by 100:

3,600 = g

So Judy used 3,600 gallons.
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The bill consisted of a fixed charge of $13.50 plus a charge of $0.0075 per gallon for the water used in the quarter.
Let G = number of gallons Judy used

So, we can write: TOTAL BILL = $13.50 + ($0.0075)(G)

Judy was charged $40.50, so we can write: $40.50 = $13.50 + ($0.0075)(G)
Subtract $13.50 from both sides to get: $27 = ($0.0075)(G)
Solve: G = 27/0.0075 = 3600

Answer: 3600

Cheers,
Brent
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